1. Field Of The Invention
The present invention relates generally to circuit boards and more specifically to multilayer printed circuit boards and an improved method for fabricating such boards.
2. The Prior Art
A very common form of printed circuit board uses a fiberglass/epoxy substrate having copper foil on one or both sides. Typically, a photoresist is applied to the copper, then the coated board is exposed with the desired circuit pattern. The board is then etched to remove the unwanted copper, leaving solder pads for connection to the components interconnected by circuit traces for providing power and signals to the electronic components. If connections between the conductor patterns on the two sides of the board are necessary, they are usually achieved by providing plated through holes, i.e., holes drilled and electrolytically plated to connect conductors on opposite sides of the board.
The miniaturization of electronic components and the increasingly dense circuit packing that it has fostered, has imposed very stringent demands on printed circuit board technology. All too often it is not possible to form all the necessary connections between densely packed circuits using just two sides of a printed circuit board. In those cases, it is often typical to resort to multilayer circuit boards.
Conventional multilayer circuit boards can be conceptualized or made up of multiple units of more or less standard glass/epoxy copper clad boards. Typically, after circuit patterns are formed on the boards by conventional etching techniques, the individual boards are laminated together. Attention must be given to making connections between selected ones of the multiple layers of circuit patterns, often using ferrules, vias or plated through holes between appropriate layers of the laminated structure. U.S. Pat. Nos. 3,436,819 and 4,388,136 illustrate certain techniques used in producing conventional multilayer circuit boards, as well as some of the problems there involved.
While the multilayer circuit board has provided the ability to distribute interconnections among a number of isolated planes, thereby to allow the dense packing of complex microelectronic circuit elements, that provision has not come without its difficulties. The primary drawback is cost. The individual boards which are to make up the laminate must be drilled, exposed, etched and assembled to rather rigorous standards, since when laminated together, they must be in precise registration with each other. A number of techniques have been developed for making interconnections between selected points in the conductive pattern on various layers, but none are without their own characteristic difficulties. Laminating the various layers together can introduce further complications. The end result is to take double-sided printed circuit board technology, itself a complication over single-sided technology, and multiply the level of complexity and therefore the cost.
It has been proposed, in order to avoid the complexity of conventional multilayer circuit board techniques, to utilize a conventional printed circuit board and to apply to that board selective interconnections by way of deposited traces of conductive resin. If more than one layer of conductive traces is desired, the layers (or the individual traces), can be separated by an insulating layer. Such an approach is illustrated in Cayrol U.S. Pat. No. 4,187,339.
While that approach facially has the advantage of significantly reducing the cost of a multilayer printed circuit board, it is not believed to have achieved commercial acceptance for a number of reasons. The conductive resins disclosed in that patent are believed unsuitable for use with today's printed circuit board technology. The structure disclosed in that patent appears to be incompatible with dip soldering techniques for plating the conductive copper, or wave soldering techniques for final soldering of components on the board. For those reasons, it appears that all of the conductive and insulative layers applied to the board were applied to the component side, leaving copper pads cn the other side for making solder connections to circuit components. That requirement dictates that all boards using that technique be double sided with plated through holes. Even with all of the polymeric traces confined to one side of the board, steps must be taken to protect that side during soldering to prevent damage to the polymeric conductors. Finally, polymer circuit boards have been avoided in the past because of the large quantity of conductive polymer needed to make the traces sufficiently conductive, and due to the historical inability of the polymers to remain bonded to the board in their desired location.